This invention generally relates to an ink jet recorder of the charging deflection type in which after ink droplets emerging from a nozzle are charged, they are deflected in a predetermined direction by the application of electric field so as to form recording dots on a recording medium. More particularly, this invention relates to an improvement of the micro-dot ink jet recorder in which two kinds of ink droplets of large diameter and small diameter are alternately emitted from a nozzle and only the ink droplets of small diameter are used for recording.
In an ink jet recorder, ink droplets are emitted from a nozzle by applying a high frequency excitation voltage on a piezoelectric device mounted to the nozzle. The produced ink droplets are varied in their form by controlling the excitation voltage applied to the nozzle. For example, three forms of the ink droplets due to different excitation voltages are disclosed in U.S. Pat. No. 4,050,077 to Takahiro Yamada et al. assigned to the same assignee as this application.
For realizing a high quality image in the ink jet recorder, it is most important that the ink droplets have uniform diameters. Nevertheless, in actual practice, small diameter ink droplets will follow larger diameter normal ink droplets, which will degrade the recorded image quality. Therefore, in the prior art, the excitation voltage was controlled so as to not produce such small diameter ink droplets, and so the small droplets have not been used for recording in the ink jet recorder.
Yamada, who is one of the inventors of the present invention and others proposed to use, for recording, these small diameter ink droplets which conventionally have been avoided. This is because the small diameter ink droplets provide small dots, which permits the recording to be more precise and the gradation of the recorded image to be more minute. U.S. Pat. No. 4,050,077 mentioned above discloses a micro-dot ink jet recorder in which only small diameter ink droplets between the ink droplets of small diameter and large diameter are used for recording, i.e., only the small diameter ink droplets are selectively charged and deflected. Further, U.S. Pat. No. 4,408,211 to Yamada discloses a method for carrying out the charging and deflection of the small diameter ink droplet by means of a common electrode in a micro-dot ink jet recorder. The micro-dot ink jet recorder using small ink droplets is a remarkable invention in that the small ink droplets can be produced without reducing the diameter of the ink jet nozzle. Incidentally, a physical mechanism in which both large diameter ink droplets and small diameter ink droplets are emitted from the nozzle has not been clarified sufficiently as yet, but Yamada et al. experimentally found the condition of the excitation voltage for assuring the alternate production of both large and small diameter ink droplets with uniform diameters as shown in U.S. Pat. No. 4,050,077.